The present disclosure relates generally to the field of kinetic energy storage. More specifically, it relates to flywheel energy storage for stationary applications where cost is of high importance; in some cases, of higher importance than weight. These applications include frequency regulation, time-of-use, uninterruptible power supply (UPS), demand response, and smoothing of renewable energy generation sources, among other applications.
Flywheels have been used as energy storage devices or for smoothing mechanical or electrical power for hundreds of years. Recently, there have been significant advancements in the field of flywheel energy storage because of the availability of high strength-to-weight (the specific strength) materials, like composites. The kinetic energy stored per unit mass of flywheel material can be shown to be directly proportional to the specific strength (strength divided by density) of the material. Because some composite materials have very high specific strength, composites make attractive candidates for flywheels having a high energy storage potential per unit mass. As an example, a high-strength carbon fiber composite (e.g., T700 at 70% volume fraction) has a fracture strength of 3430 megapascals (MPa), or 490,000 pounds per square inch (psi) and a density of 1845 kilograms per cubic meter (kg/m3), or 0.067 pounds per cubic inch (lb/in3). Compare that to a non-composite material, such as a high-strength alloy steel, which has a yield strength of 1400 MPa (200,000 psi) and a density of 7870 kg/m3 (0.285 lb/in3). On a strength-to-weight basis, therefore, composites have more than ten times higher specific strength and, therefore, are able to store more than ten times the energy per unit mass compared to steel. This potential has led inventors to pursue designing flywheels based on composite rotors.
However, composite materials have not been cost-effective in stationary applications (i.e., applications in which weight is not the primary concern) where the primary goal is maximum energy stored per unit cost, rather than maximum energy stored per unit weight.